1. Field of the Invention
The present invention relates to a process for the detection of phase boundary surfaces between layers of different fluids which are not intermixable with one another, and which can be separated from one another by gravity separation, and to a measurement apparatus for the performance of the process.
2. Background Information
Various processes exist for the detection of phase boundary surfaces between layers of dissimilar fluids which are not mixable and which can be separated by gravity separation. The pressure level and the specific gravity of the different fluids can be measured, and the phase boundary surface can be calculated by determining the ratio of the specific gravities and normalizing the result to the measured pressure level. Such processes may prove unsatisfactorily if, for example, the specific gravity of one of more of the participating fluids fluctuates significantly, if the specific gravity cannot be determined with sufficient accuracy, or if the measurements can be distorted by the type of process control in the container containing the liquids, which can occur, for example, as a result of the superimposition of external pressure fluctuations on account of acceleration processes within the fluids, or as a result of other external hydrostatic influences.
There also exist conductive measurement methods, in which the electrical conductivity of the fluids is measured, if there is a significant difference between the electrical conductivity of the different fluids. These methods have the disadvantage that at least one of the fluids covers the sensor electrodes on account of its adhesion and viscosity, and there is no automatic, process-controlled cleaning, so that the measurement values may be distorted.
There also exist capacitive measurement methods, in which the dielectric constant is measured in the presence of at least one electrically non-conducting fluid. This measurement process fails if the conductivity of the non-conducting phase exceeds the value of 10 to 50 milli- Siemens, for example. Such a situation can occur if two fluids which normally cannot be mixed with one another form an intermediate phase in the form of a stable emulsion. Such an emulsion can occur, for example, in a multi-phase system such as water (e.g., seawater) and organic fluids. In the presence of a natural or synthetic emulsifying agent, a stable emulsion can be formed. The boundary layer between such an emulsion with, for example, a seawater proportion of more than 50%, which floats on account of its lower specific gravity, and the free water in the sump of a container or apparatus cannot be detected by capacitive measurement, on account of the fact that the conductivity of the emulsion is similar to that of the water.
It is also possible to monitor the movement of a phase boundary surface by measuring the specific gravity. A prerequisite for such monitoring is that there must be significant differences between the specific gravity parameters. This process cannot be used if viscosities which are significantly different from that of water, for example, up to several thousand centi-Stokes, must be processed, since the reaction of the measurement apparatus over time is very strongly influenced by the self-cleaning characteristics of the sensor, which has a conventional tubular shape and carries the flow.
Other viscosity measurement processes which exist require that there must be significant differences in the viscosities of the different fluids. In many cases, however, the viscosity values of different fluids which cannot be mixed and can be separated by gravity separation are very similar, such as for water and light oils or petroleum from certain producing regions, for example.
To detect phase boundary surfaces, there also exist sonar processes, by means of which the speed of sound in fluids is measured perpendicular to the phase boundary surface. For example, sound waves are transmitted, preferably from below, by a transmitter toward the phase boundary surface. The sound waves are reflected by the phase boundary surface. The propagation time or "echo time" of the sound waves is then evaluated as a yardstick for the distance of the phase boundary surface. The receiver for the reflected sonar signals can be located in the immediate vicinity of the sensor. It is also possible to configure the sensor and receiver as acoustic transformers. Since the sonar process is based physically on a distance measurement, it cannot be used (e.g., will not necessarily yield accurate results) if a significant proportion of the sound waves are reflected diffusely on the phase boundary surface. Such a diffuse reflection occurs if a fluid phase represents an emulsion having a viscosity which is significantly higher than the other fluid, or if the phase boundary surface permanently changes its structure and position on account of flow (e.g., current) factors or movements of the container, which occurs, for example, in the tank of a ship.
In order to overcome the above-noted disadvantages of the measurement processes described above, attempts have been made to use measurement devices with different sensors, by means of which several of the measurement processes described above can be combined. Such multi-sensor arrangements, however, are technically very complex and require a very complex evaluation system. Accordingly, the use of such multi-sensor measurement devices is very expensive.